Re: Dot shape for web presses
I agree with Dan that the best dot shape for an AM screen for offset (web or sheetfed) in a CTP environment is Round. The choice of dot shape is important because it has an impact on the aesthetics of the halftone as well as its lithographic performance on press. This is especially true for lower frequency screens in the 85-175 lpi range.
When dots grow in size from highlight to shadow there is a point at which the dots first touch each other. When that happens there is an effect called the “optical bump.” In a gradient blend this shows up as an artifact of a dark line in an otherwise smooth blend. Another issue is that, because paper moves through a press from printing unit to printing unit, presses are directional imaging devices which means that dots that are also directional can interact with the directionality of the press and cause artifacts. Directional issues on press include slur, doubling and tailing (slinging).
Here are the basic dot shapes in use - every vendor will have these options with very subtle variations:
Round dot (a.k.a. non-transforming dot) ; Dots are round through the tone range. They start as a point and simply grow equally in all directions through the tone scale.
Benefits: Dot shape is the same for all screen angles and frequencies, optical bump is hidden in the shadows at the 75% tone, dot is non-directional so it is less affected by press problems. Excellent for computer-to-plate imaging because of the greater integrity of halftone imaging as well as the ease of dot gain compensation with tone reproduction curves.
Issues: Not suited for film-imaged plates because the diamond shape that results at 75% and darker tones is very sensitive to dot gain and sudden loss of shadow detail.
Euclidean dot: Round/Square/Round (a.k.a. transforming or composed dot) Dots begin round at 1% and transform to square at the 50%. Shadows from 50% to 99% are a negative image of the 1%-50% tone scale.
Benefits: Dot shape, except for 50% tint, is the same for all screen angles and frequencies, dot is non-directional so it is less affected by press problems.
Issues: optical bump occurs at 50% midtone tint - very visible in vignettes.
Elliptical dot: Rounded corner diamond/elliptical shape
Benefits: optical bump is moderated by being split into two – when the dots first touch at the long width at the 40% tint and then again at the short width at 60%.
Issues: dot shape varies at different screen angles which can cause single color moiré and uneven dot gain. Dot is directional, at low lpi frequencies the “chaining” of the dots as two points touch can cause artifacts that appear as lines. Directional problems on press such as slur and doubling can cause strong tone and color shifts depending on the angle of orientation of the dots relative to the angle of the paper as it travels through the press.
best, gordo
